August 28, 2026 · 7 min read
Net peptide content, salt form, and why a 10 mg vial isn’t 10 mg of peptide
Purity and content are different questions, and almost nobody publishes the second one. The gap between them is usually 10–30% of the mass you paid for.
A vial labelled 10 mg with a certificate reading 99.2% purity looks unambiguous. It is not. Purity and net peptide content answer different questions, and a vial can be genuinely 99% pure while containing meaningfully less than 10 mg of peptide. This is not a scandal and it is not usually deception — it is chemistry that most suppliers simply never explain.
Where the missing mass goes
Synthetic peptides are purified by reverse-phase HPLC, and the mobile phase almost always contains trifluoroacetic acid. TFA is an excellent ion-pairing agent, which is why it is used, and it does not entirely wash out. Basic residues — lysine, arginine, histidine, and the free N-terminus — retain TFA counter-ions through lyophilization. The powder in the vial is therefore a peptide salt, not free peptide.
Two other components share the mass. Lyophilized powder retains bound water, typically a few percent, which is what Karl Fischer titration measures. And where a peptide has been exchanged into an acetate salt, acetate takes the place of TFA at a different mass. Add these together and the gross powder mass routinely exceeds the peptide mass by 10–30%.
Why purity does not capture this
RP-HPLC purity is a ratio of peak areas at a UV wavelength that detects the peptide bond, usually 214 or 220 nm. Counter-ions, water and salts do not absorb meaningfully at those wavelengths, so they never appear as peaks and never reduce the purity figure. A 99% pure peptide is 99% pure relative to other UV-absorbing material — which is a statement about the peptide's chemical homogeneity, not about how much of it is in the vial.
Both figures are true simultaneously. “99% pure” and “roughly 80% net peptide by mass” describe the same vial without contradiction, and a supplier quoting only the first is not lying — it is answering the easier question.
The method that does capture it
Net peptide content is determined by amino acid analysis: the peptide is hydrolysed to its constituent amino acids, which are quantified against standards. The figure it returns is the proportion of the powder mass that is peptide. It is a slower and more expensive assay than HPLC, which is why it is rarely run on routine commercial lots, and why publishing it is a genuine differentiator rather than a checkbox.
What this means when comparing suppliers
Two vials at the same nominal mass and the same quoted purity are not necessarily comparable. If one is a TFA salt at 78% net peptide and the other has been exchanged to acetate at 89%, the second contains materially more peptide per milligram of powder. Neither certificate is wrong. The comparison simply cannot be made from purity alone.
| Figure | What it measures | How it is determined |
|---|---|---|
| Purity | Proportion of UV-absorbing material that is the target peptide | RP-HPLC, area percent at 214 or 220 nm |
| Identity | That the molecule is the one named | Mass spectrometry against theoretical mass |
| Net peptide content | Proportion of powder mass that is peptide | Amino acid analysis after hydrolysis |
| Water content | Residual moisture in the lyophilized cake | Karl Fischer titration |
| Counter-ion | Salt form and its share of the mass | Ion chromatography, or stated from the purification method |
The practical position: treat purity as a statement about homogeneity, treat net peptide content as a statement about quantity, and do not let one stand in for the other when you are comparing prices per milligram.